A listing in one of Azure networks reveals following address space ace::/64
What it is?
It's the short form for an IPv6 address. The extended form would be ace:0:0:0:0:0:0:0, however, blocks with 0 can be shortened with the double colon, so the result is ace::. The /64 means, that the address block marked by this address contains 2^64 addresses (an IPv6 address has 128 bits, and in this case 64 bits describe the network, and the other 64 can be assigned to actual devices in the network).